The field of the present disclosure relates generally to forming moldable articles and, more specifically, to forming ceramic cores that may be used in an investment casting process.
At least some known metallic turbine components, such as blades, nozzles, and vanes, have complex internal and external geometries. For example, turbine blades and nozzles may have internal passages and/or voids defined therein that may be used for cooling purposes. These passages must be manufactured in accordance with accurate dimensions having tight tolerances. In such instances, investment casting is generally effective at manufacturing parts that require precise dimensional accuracy.
Manufacturing metallic turbine components generally requires the fabrication of a ceramic core that acts as a pattern and defines the internal cooling passages. Ceramic cores may be fabricated using any suitable ceramic processing method. Generally, ceramic powder is mixed with binders and/or volatile liquids to form a slurry or plastic mixture. The mixture is then formed into a desired shape and cured in a molding process, such as an injection molding process. The formed green ceramic article is then subjected to one or more heat treatments to remove the volatile components and to sinter the ceramic material. The ceramic core may then be used in an investment casting process.
However, firing the green ceramic core generally results in shrinkage and deformation of the core such that the core dimensions may fall outside of acceptable tolerances. For example, a green ceramic core may be subjected to both sinter firing and setter firing processes. Sinter firing includes heating the ceramic core in a bed of sand or setter base, resulting in a partially-densified core that has been shrunk and deformed. The core is then setter fired using a two-piece (base and lid) setter in an attempt to use the principle of temperature creep to mold the core back within an acceptable tolerance range. However, the two-piece setter may result in core breakage in cases of extreme deformation. Core breakage may be partially remedied by positioning a number of wooden shims between the setter lid and base. The molding apparatus is then fired in a furnace such that the ceramic material reaches its glass transition temperature as the wooden shims burn away. Accordingly, the setter lid gradually moves into a desired molding position on top of the core. However, burning wood leaves a quantity of ash that facilitates preventing the lid and base from coming into direct contact with one another thereby creating a dimensionally inaccurate article.